Electrically adjustable items of furniture are increasingly being offered. Thus, in the case of many tables, especially writing tables, the height of the table itself can be electrically adjusted by a special drive. Beds, too, for example luxury beds or hospital beds, can be adjusted somewhat in height or angle via electrical drives. However, other electrical consumers are increasingly being found in furniture, for example a heating mat or vibrating motors in beds or luxury armchairs.
In the case of electrically adjustable furniture it is possible to use DC motors that can be operated with low voltage for the drive of the electrical adjusting device. For this the line voltage is stepped down, for example, by means of a transformer, to the lower voltage range of the DC motors and converted to DC voltage via a rectifier circuit. Control of the DC motors can take place by a controlled connection of the motors to the DC voltage thus generated. Alternatively, control can also be provided with a DC voltage that corresponds to the lower voltage range of the motors via a switchable power supply.
For example, a switchable power supply can be incorporated in a plug type power supply unit. Since the same AC line voltage is not used in all areas of the world, it may be necessary to make available different switchable power supplies for different line voltages. For instance, in most of Europe a line voltage of about 230 volts is available, while in North America the line voltage is about 110 volts. So that a switchable power supply can be used in both voltage regions, it is possible to provide an electronic detection circuit that recognizes the voltage range that is present in each case and switches the switchable power supply to an appropriate operating mode. However, such a detection circuit increases the production cost and thus the costs of the switchable power supply component.
Furthermore, a specific voltage range in which the switchable power supply is operated can be chosen by means of a manual switch. However, if there is erroneous operation of the manual switch or erroneous choice of the voltage range, this can lead to damage or destruction of the switchable power supply.